How do I clear the render border?
Use Ctrl + Alt + B to clear the render border, or if you open the Space menu and type "render border", you should see the option.



There is also Shift + B, but this is the legacy shortcut for border render IIRC and it only works when looking through the camera. Use Ctrl + B as you can have different render borders for the viewport (when doing realtime rendering) and the camera.